A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The surface of a substrate on which a pattern should be projected is usually not completely flat. Moreover, a substrate can show thickness variation of several microns. This flatness and/or thickness variation of the substrate surface may result in incorrect projection of the pattern, for instance due to focus errors or imaging errors.
To correct for unflatness and/or thickness variation of a substrate, it has been proposed to provide a level sensor, desirably integrated in the lithographic apparatus. Such a level sensor may be used to determine a height map of a substrate, before a pattern is transferred, e.g., projected, on the substrate. This height map may subsequently be used to correct the position of the substrate during transfer of the pattern on the substrate.